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c oncurrent with that of the engines themselves. The earliest flights were
m ade without any engine instruments at all. Early instruments were chiefly
for the purpose of indicating whether or not the engine was performing
satisfactorily (in fact this is still the purpose of most engine instruments
c arried in aircraft). First to come into use was some sort of tachometer for
o bserving engine speed. It was followed, in the approximate order of use,
by sight glasses or gauges to indicate oil flow or oil pressure, remote-reading
t emperature gauges for oil and coolant, and thermocouples for indicating
t he temperature of air-cooled cylinders at some critical point. As flight
d uration increased, fuel-supply indicators, usually showing fuel-level in
t anks, w ere found important. The introduction of supercharging required
m anifold-pressure i ndicators. A late development in engine instruments
is the engine "analyzer," an electronic system which observes the ignitionvoltage versus time curve of any cylinder on a cathode-ray screen and which
r equires the services of a flight engineer. Used in most large multi-engine
a irplanes, it enables a trained observer to detect and anticipate many forms
of engine trouble or incipient failure by observing these curves for each
c ylinder in turn.
E NGINE INSTRUMENTS. A d etailed account of instrument history and technology is beyond
t he scope of this volume. Chatfield, Taylor, and Ober in the various
e ditions of The Airplane and its Engine (1928-1948) give descriptions of instruments as they appeared at the time of publication. Other relevant
p ublications will be found in the bibliography. 83 400 o
LO en Ui
LO GO 300 LO
O X <
GO DO cr.
LO GO 1900 1910 1920 1930 1940 1950 1960 F igure 7 1 . — E ngine d e v e l o p m e n t curves. Typical performance characteristics of m i l i t a r y and
l arge c o m m e r c i a l airplane piston engines, 1 9 0 3 - 1 9 6 0 . From 1930 on these curves apply to
s upercharged e n g i n e s ; unsupercharged engines, except as regards overhaul period, remain
a t a p p r o x i m a t e l y the 1930 levels. 84 S ummary of Piston-Engine Development
F igure 71 shows performance parameters for piston aircraft engines since
B rake mean effective pressure ( bmep) is a measure of an engines'
a bility to withstand high cylinder pressures and to produce power with a
given speed and size. Starting at 62 psi (Langley, 1902), it rose to 130 by
1925, w hich is near maximum for unsupercharged engines. With the
i ntroduction of supercharging and improved fuels in the 1930s, bmep was
i ncreased to takeoff values up to 360 psi (Rolls-Royce Merlin) and 300
psi (large radial engines in the United States), where it has remained since
t he advent of jets and turbines.
M ean piston speed (mps) a t takeoff rose steadily from 750 ft/min i n
1903 to a maximum of 3,000 ft/min in 1935, w here it has remained.
Specific fuel consumption has been reduced from nearly 1 l b/hp-hr...
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